A permanent magnet motor having a rotor with permanent magnets has been used as home appliances, industrial machines, automotive machines and information machines because of its high efficiency. The permanent magnet motor has been used as a motor driving a compressor and a fan of a heat exchanger and so on. A motor control unit executes vector control to divide a current into a d-axis component indicating a magnetic flux component and a q-axis component indicating a torque component perpendicular to the d-axis component and to control the d-axis component and the q-axis component respectively.
The motor control unit executes sensor-less control to detect a rotational position of the rotor of the permanent magnet motor when the permanent magnet motor is used in a heat environment or a high pressure environment in order to secure high efficiency. JP 2007-181352A discloses a motor control unit that estimates a rotational position based on the d-axis current, the q-axis current and the d-axis voltage, or the d-axis current, the q-axis current, the d-axis voltage and the q-axis voltage.
This motor control unit estimates the rotational position based on an induced voltage generated by rotating the permanent magnet motor. JP 2006-129663A discloses a motor control unit that rotates a permanent magnet motor by increasing the d-axis current from zero and maintaining the q-axis current at zero and then starts a forcible commutation during a start-up operation without generating an induced voltage.
There is a problem that start-up time becomes long because it takes long time for an oscillation of the rotor to converge and for the rotor to stop at a proper position, and for the motor control unit to start a forcible commutation if a moment of inertia is large such as the case for a fan motor of a heat exchanger of an air conditioner. To avoid this problem, the forcible commutation may be started before the oscillation of the rotor converges. However, this often results in a start-up failure.